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Biomedical subjects

B N Cuffin

Publications and source records attributed to B N Cuffin.

34 records · Page 2Linked to original sources
PubMed1985

Electrical sources in human somatosensory cortex: identification by combined magnetic and potential recordings.

Magnetic fields and electrical potentials produced by neuronal activity have different properties that can be used for the identification of electrical sources in the human brain. Fields and potentials occurring 20 to 30 milliseconds after median nerve stimulation in human subjects were compared in order to investigate the sources of evoked potential components that have been attributed by different investigators to the thalamus or thalamocortical afferents, to separate radial sources in somatosensory cortex and motor cortex, or to a tangential source in somatosensory cortex. The magnetic and potential wave forms were highly similar in morphology, and their spatial distributions were centered over sensorimotor cortex, were dipolar in shape, and differed in orientation by approximately 90 degrees; distances between the minimum and maximum of the magnetic distributions were about 60 percent of those of the potential distributions. These results cannot be accounted for by thalamic sources or radial cortical sources alone, but are consistent with a tangential source in somatosensory cortex, with an additional smaller contribution from radial sources.

Electrophysiology↗
PubMed1983

Demonstration of useful differences between magnetoencephalogram and electroencephalogram.

For a dipole source, theory predicts 3 useful differences between the MEG and EEG spatial patterns over the head. These are seen when a comparison is made between theoretical MEG and EEG maps, due to the dipole in a spherical model of the head. If true, these differences would allow the MEG to better localize or differentiate neural sources in some ways than does the EEG. A first experimental test of the differences is made here. A comparison is made between MEG and EEG maps due to a neural source which appears to behave as a dipole (N20 of the somatic evoked response). The same 3 differences are seen, therefore the predicted differences are confirmed experimentally. The first 2 differences, due only to the tangential component of the dipole, are that the MEG pattern is rotated by 90 degrees from the EEG pattern and is one-third tighter. The first allows the MEG to localize a tangential dipole better in a preferred direction, across the dipole (while the EEG does so along the dipole); the second allows the MEG to localize somewhat better in its preferred direction than the EEG does in its preferred direction. The third difference is due only to the radial component of the dipole; while the MEG receives no contribution from this component, the EEG pattern is asymmetrical and is heavily weighted by it. This allows the MEG to reveal tangential sources which are obscured by the radial sources in the EEG. For sources which cannot be approximated by a dipole, the MEG-EEG differences will depend on the particular case; however, the spherical model can now be used with more confidence to predict differences in these cases.

Brain↗
PubMed1983

Magnetic measurement of S-T and T-Q segment shifts in humans. Part I: Early repolarization and left bundle branch block.

The direct-current magnetocardiogram not only shows the features usually seen on the electrocardiogram, but also shows the T-Q (baseline) shift due to cardiac injury current. The first direct-current magnetocardiogram measurements of the human heart are presented here. The hypothesis tested is that there is no injury current associated with the S-T shift seen in two electrocardiogram abnormalities: early repolarization, and left bundle branch block. The data from three typical early repolarization subjects and one typical left bundle branch block patient are presented. It is found, in each case, that although there is appreciable S-T shift, there is essentially no baseline shift on the direct-current magnetocardiogram. The absence of baseline shift proves that the S-T shifts in these cases are not "apparent" shifts, caused by a dc injury current which is interrupted during the S-T interval; instead, these are "true" S-T shifts caused by a current flowing only during systole, presumably due to an altered repolarization of the ventricles. It is also found that the direct-current magnetocardiogram does not have routine clinical application because of a practical problem. This is the presence of false baseline shifts due to noncardiac currents, mostly in the gastrointestinal tract, which could be suppressed in only about one-third of the subjects. However, the direct-current magnetocardiogram may be useful as a research tool, for clarifying the cause of the S-T shift in selected subjects.

Electrocardiography↗
PubMed1980

Magnetic fields produced by steady currents in the body.

The magnetic fields produced by naturally occurring steady currents in the body were measured by using a new magnetic gradiometer in a magnetically shielded room. A field of 0.1 micro G/cm with reproducible pattern was seen over the head and over the limbs, whereas the field over the torso proper was weaker (except over the abdomen). Most of the field over the head is produced by electrical sources associated with the hair follicles of the scalp; this field is produced only as a response to touching or pressing the scalp, in regions where the hair is dense. Most of the field over the limbs is produced by electrical sources associated with the muscles. The field over the forearm, studied in detail, was often present spontaneously; when absent, it could be induced by mild twisting and rubbing. On the basis of auxiliary experiments involving electrolytes, a general mechanism for generation of steady current in the body is suggested. In this mechanism, the steady current is generated by a nonclosed or a nonuniform polarized layer across an elongated semipermeable membrane such as a muscle fiber; the nonuniform polarization is due to a gradient of extracellular K+ along the membrane.

Abdomen↗
PubMed1979

Comparison of the magnetoencephalogram and electroencephalogram.

The spatial response of the magnetoencephalogram (MEG) to sources in the brain's cortex is compared with that of the electroencephalogram (EEG). This is done using computer modeling of the head which is approximated by 4 concentric spherical regions that represent the brain and surrounding bone and tissue. Lead fields are calculated at points on the cortex for unipolar, bipolar and quadrupolar MEG and EEG measurements. Since lead fields are patterns of the sensitivity of these measurements to a source at various locations and orientations, they provide a convenient means for comparison. It is found that a unipolar MEG has a very different lead field than a unipolar EEG. Hence, this type of MEG detects sources at different locations and orientations than this EEG. Although bipolar MEG and EEG lead fields are found to have similar patterns, the MEG lead field is narrower than that of the EEG and hence 'sees' a smaller area on the cortex than the EEG. This is because the potentials measured by the EEG are 'smeared' by the low-conductivity skull; the magnetic fields measured by the MEG are not smeared. Quadrupolar MEG and EEG lead fields are found to be about the same. The responses of bipolar MEGs and EEGs to distributed sources, which are composed of aligned and randomly oriented dipoles, are compared. It is found that for both types of sources, the MEG 'sees' an area on the cortex which is approximately 0.3 times that for the EEG. Hence, the MEG appears to be useful for detecting a more restricted group of sources than the EEG.

Brain↗
PubMed1976

Part III: the effect of the torso boundaries on the magnetocardiogram.

The magnetic field produced by a current dipole is made up of two parts: the field from the dipole element, and from the current generated by the dipole in the volume conductor. It was previously shown for the semi-infinite volume conductor, infinite slab, and the sphere that the volume-current contribution is zero to the component of magnetic field which is normal to the boundary. The volume conductor in the form of the human torso is here investigated by computer simulation. Three different heart-torso models are used. The contribution to the normal field component (Bn) by the volume current (via the boundaries) and by the heart dipoles are computed. For comparison, the boundary contribution to the surface potential (V) is also computed. For Bn the three models yield a ratio of boundary to dipole contribution in the same range, with 0.28 as the average. Simple subtractions can make this ratio negligible. For V the equivalent ratio is somewhat greater. The arrow map, developed previously to display Bn over special surfaces, is shown to be valid for the human torso, for visually estimating the heart dipoles.

Electromagnetic Phenomena↗